The drivetrain of a hybrid vehicle includes two combined power sources for the propulsion of the vehicle. Usually an internal combustion engine is combined with an additional power source connected to or including an energy storage unit which can be charged by the internal combustion engine, and, in many solutions, also by recovering the braking energy of the vehicle.
In so called hybrid electric vehicles, the additional power source is typically an electric motor connected to an energy storage unit including rechargeable electric accumulators for energy storage. In alternative hybrids, the additional power source could include, instead of electric components, a variable displacement pump communicating with a hydraulic accumulator for energy storage, essentially in the form of a pressure tank. There are also hybrid concepts in which the additional power source includes a flywheel which stored energy and is connected to a hydrostatic transmission. In further hybrid ideas, the additional power source includes an air engine connected to an air tank for energy storage. The additional power source can also be a human, such as in the case with a moped.
The drivetrain of a hybrid vehicle can be provided as a parallel hybrid arrangement, in which the engine and the additional power source (e.g. electric motor) are both connected to a mechanical transmission for delivering torque to the wheels. Where the additional power source is an electric motor, this is often provided as a combined generator and motor. Differing from parallel hybrid arrangements, in a series hybrid arrangement there is no power path from the engine to the wheels. The main task of the engine is to provide power to the additional power source and/or the energy storage unit connected thereto. Combined hybrids have features from both parallel and series hybrid arrangements, in that they have power split devices allowing the power path from the engine to be mechanically directed to the wheels, or to be directed to the additional power source or the energy storage unit connected thereto.
The operation of most hybrid vehicles with internal combustion engines usually includes a number of different modes for the power distribution in the drivetrain. For example, the vehicle can be run in a cruise mode, in which the power from the engine is split into a path to the wheels and a path to the energy storage unit, e.g. to batteries via a generator. As another example, the vehicle can be run in an energy storage unit charge mode, (for a hybrid electric vehicle usually referred to as a battery charge mode), during engine idling, in which the energy storage unit is charged by the engine, e.g. via a generator. In parallel or combined hybrid arrangements, there could also be a power boost mode in which power is provided to the wheels from both the engine and the power source.
In addition, the operation of most hybrid vehicles with internal combustion engines usually includes engine off modes, in which the internal combustion is shut down. Such modes can include a mode in which propulsion is provided only by the additional power source. An example of such a mode is a so called electric vehicle mode in a combined or parallel hybrid electric vehicle. For the vehicle to run in an engine off mode, necessary accessories are powered by the additional power source.
Usually, one or more exhaust gas aftertreatment devices, known as catalytic converters, are provided in the exhaust system of the engine. Catalytic converters convert regulated gases, such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), into substances such as carbon dioxide (CO2), nitrogen (N2) and water (H2O). A catalytic converter can not fully exhibit its converting effects when its temperature is lower than its activation temperature. A problem with hybrid vehicles is that during engine off modes, the catalyst converter temperature can fall below the activation temperature, in which case HC and NOx emission are not efficiently processed in the exhaust gas aftertreatment devices when the internal combustion engine is started again.
For vehicles in general, many catalyst heating strategies have been suggested. U.S. Pat. No. 6,161,377 and U.S. Pat. No. 6,901,749 are concerned with rapidly heating the catalytic converter during cold starts. EP0935056A2 describes shutting off ignition in one or more cylinders when cranking during engine start, to heat the cold catalyst. EP1174612A1 describes, in a diesel engine application for regeneration of a particulate filter or a NOx-trap, injecting fuel into the cylinders during an engine braking situation. However, none of these references describe hybrid vehicles.
US2004/0194452A1 discloses a hybrid vehicle where fuel supply is carried out during an initial period of cranking for resuming the operation of the internal combustion engine. U.S. Pat. No. 6,318,077B1 describes a special heater for a catalyst for rapid heating of the catalyst at a cold start. However, none of these references describe maintaining catalyst temperature during an engine off mode.